An ordinary air cooling type small two-stroke gasoline engine which is conventionally used in a hand held type portable power working machine such as a chain saw and brush cutter is constructed such that an ignition plug is disposed at the head portion of the cylinder. An intake port, a scavenging port, and an exhaust port, which are to be opened and closed by a piston, are provided so as to communicate with the cylinder bore (or provided in the inner peripheral wall of the cylinder). According to this two-stroke internal combustion engine, one cycle of engine is accomplished by two strokes of the piston without undergoing a stroke which is exclusively assigned to the intake or exhaust of gas.
More specifically, in the ascending stroke of the piston, an air-fuel mixture consisting of a mixture comprising air, fuel and lubricant is introduced from the intake port into the crank chamber disposed below the piston. Then, in the descending stroke of the piston, the air-fuel mixture is pre-compressed in the crank chamber producing a compressed gas mixture, which is then blown from scavenging port into a combustion actuating chamber which is disposed above the piston, thereby enabling waste combustion gas to be discharged from the exhaust port. In other words, the scavenging of the waste combustion gas is effected by making use of the gas flow of the air-fuel mixture.
Therefore, the unburnt air-fuel mixture is more likely to be mingled into the combustion gas (exhaust gas), thus increasing the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged into air atmosphere without being utilized for the combustion. Because of this, as compared with a four-stroke engine, the two-stroke internal combustion engine is not only inferior in fuel consumption but also disadvantageous in that a large amount of poisonous components such as HC (unburnt components in a fuel) and CO (incomplete combustion components in a fuel) are included in the exhaust gas. Therefore, even if the two-stroke engine is small in capacity, the influence of these poisonous components on the environmental contamination should not be disregarded. Additionally, there are several problems as to how to address the regulation of exhaust gas which would become increasingly severe from now on. In particular, there are difficulties as to how to deal with the minimization of HC (total HC) in the exhaust gas.
With a view to overcome these drawbacks, there have been proposed various kinds of countermeasures. For example, as disclosed in JP Patent Laid-open Publication (Kokai) No. 9-125966 (1997), there has been proposed a two-stroke internal combustion engine of so-called air pre-introduction type (or stratified scavenging type) wherein an air inlet passageway for delivering external air to a scavenging passageway is installed, thus enabling the air to be introduced into the combustion actuating chamber in advance to the introduction of air-fuel mixture in the descending stroke of piston. Because of this structure, a layer of air is enabled to be formed between the waste combustion gas to be discharged and unburnt air-fuel mixture. Due to this air layer, the air-fuel mixture is prevented from being mixed with the waste combustion gas, thus making it possible to minimize the quantity of blow-by of air-fuel mixture.
Further, another type of two-stroke internal combustion engine of air pre-introduction type (or stratified scavenging type) is proposed in JP Utility Model Laid-open Publication (Kokai) No. 57-53026 (1982), wherein a sub-scavenging port which is designed to be opened prior to the opening of the main scavenging port is installed, and through this sub-scavenging port, air is supplied to the combustion actuating chamber by making use of a pump to be rotationally driven by a crankshaft.
According to these conventional two-stroke internal combustion engines described in these prior documents, it is possible to form a 3-ply layer consisting of a lower layer (piston side) constituted by waste combustion gas, an intermediate layer constituted by air, and an upper layer constituted by an air-fuel mixture, which are superimposed from top to bottom (vertical direction). As a result, it is possible to obtain stratified scavenging effects, to reduce the quantity of blow-by, and to improve the emission characteristics. According to these internal combustion engines of the prior art however, in order to deliver air to the combustion actuating chamber, it is necessary to install not only an air inlet passageway (generally, the air inlet passageway having a forked configuration should be installed, since one pair of right and left scavenging passageways or more than one pair of scavenging passageways are installed) but also a pump separate from and outside the main body of engine (cylinder and crankcase). Consequently, the structure surrounding the engine, inclusive of the air inlet passageway, would inevitably become complicated and heavier. Additionally, these internal combustion engines of the prior art are inconvenient in working and assembling, so that they should be further improved for suitably mounting them on a portable power working machine.
The present invention has been made under the circumstances described above, and therefore an object of the present invention is to provide a two-stroke internal combustion engine which can be manufactured at low cost without necessitating the tremendous modification of the structure thereof and which is capable of effectively suppressing the blow-by of unburnt air-fuel mixture, of improving emission characteristics, and of improving the fuel consumption and output of engine.